Method of gasifying solid fuel



Feb. 19, 1957 v I I. ROBERTS 2,782,109

METHOD OF GASIFYING SOLID FUEL Original Filed Aug. 29, I951 Top ofcoalbed /y/uq 736527:

7 IN VEN TOR.

flu, My 024% #4- WeusV;

United States Patent METHOD OF GASIFYING SOLID FUEL Irving Roberts,Meadowbrook, Pa.

Original application August 29, 1951, Serial No. 244,172. Divided andthis application March 8, 1955, Serial No. 492,917

3 Claims. (Cl. 48-206) This invention relates to a method of gasifyingsolid fuels, especially caking bituminous coal. The application is adivision of my copending patent application, Serial No. 244,172, filedAugust 29, 1951 now abandoned for Apparatus for Gasifying Solid Fuel.

In a conventional updraft gas producer, a continuous blast of steam andair is introduced into the bottom of a bed of burning fuel to produce aproduct gas consisting of .a mixture of carbon monoxide and hydrogenwith some carbon dioxide and unreacted steam. The nitrogen in the airblast appears in the product gas as an inert diluent. When coke oranthracite coal is used as the fuel, the bed is maintained five or sixfeet deep to obtain heat transfer between the product gas and theincoming fuel so that a considerable amount of heat will be recoveredfrom the product gas before it leaves the producer. On the other hand,if the fuel is a caking bituminous coal, it will begin to fuse somewherebetween about 750 and 950 F. and will form large masses of coke. Theselarge lumps are slow to react with the gases in the bed, and the risingproduct gas tends to by-pass the lumps and form channels or blowholesthrough which partially reacted gas flows and mixes with the product,thereby reducing its quality. Consequently, when such a fuel is used,the depth of the bed is reduced to about two feet so that thetemperature of the surface of the bed can be raised well above thefusion temperature of the coal. A rake or similar mechanical deviceis'arranged to pass over the surface of the bed to prevent the formationof any large masses of coke and to push coal into any blowholes whichmay appear. Satisfactory gas quality and output are obtained in thismanner, but the temperature of the product gas leaving the producer israised to about 1350-1550 F- This represents a substantial loss inthermal efficiency of the producer. Such a loss is especially importantin a synthesis process, such as the synthesis of gasoline from solidfueL'Where in order to avoid the presence of nitrogen in the synthesisgas relatively pure oxygen is used in place of air in the blast. Sincethe oxygen, which must be produced in an air distillation plant, isquite expensive, it is highly desirable to reduce the quantity consumedin the gas producer as much as possible. As the heat losses from the gasproducer require the generation of more heat in it by the addition ofmore oxygen, substantial reductions in oxygen requirements may be madeby improvements in thermal efiiciency of the producer. Thermalefiiciency, however, can not be improved as long as the product gasleaves the producer at the high temperature it has in the past whencaking bituminous coal has been used as the fuel. been used in theproduction of a synthesis gas. Instead, the more expensive coke oranthracite coal has been used in the making of gas for syntheticmethanol and synthetic ammonia. Such fuel is too expensive to use in themanufacture of synthetic gasoline, and, as pointed The result has beenthat such coal has not ice out above, the thermal efiiciency of a gasproducer using bituminous coal is too low.

It is among the objects of this invention to provide a method ofgasifying solid fuel, which has high thermal efiiciency and whicheliminates caking difiiculties when caking bituminous coal is used.

in accordance with this invention, fuel is fed to the upper ends of apair of updraft gas producer's to maintain deep beds therein. Steam andoxygen-containing gas are piped to the upper ends of the producers,while oxygen-containing gas also is piped to the bottoms of theproducers to support combustion of the fuel in them. Connected to thetop of each producer is an outlet pipe for product gas, while a conduitconnects the bottoms of the producers. Valves are provided for closingthe outlet pipes alternately, and further valves are provided fordirecting the steam and oxygen-containing gas into the top of theproducer having the closed outlet pipe at that time and for directingoxygen-containing gas into the bottom of the other producer. The productgas flowing upward alternately in the two producers transfers a greatamount of its heat to the deep beds of fuel which are cooledperiodically by the steam and oxygen-com taining gas that flows downthrough them alternately. When the fuel is caking bituminous coal, theflow of oxygen-containing gas down through the deep bed destroys thecaking properties of the coal before it reaches the zone in which itotherwise would cake.

The invention is illustrated diagrammatically in the accompanyingdrawings, in which Fig. 1 represents a vertical section through twoproducers with their connecting conduits; and Fig. 2 illustrates amodification of one of the producers.

Referring to Fig. l of the drawings, each producer A and B may be aconventional type in which a bricklined cylinder 1 has a tapered lowerend that opens into a pan 2 filled with water to form a seal. The panmay be rotated to remove ash from the producer. Of course, other ways ofremoving the ash may be used. Connected to the top central portion ofeach cylinder is a conduit 3, through which fuel can be delivered to it.For convenience the two conduit are shown connected to a single supplyconduit 4. The cylinders are much taller than those used heretofore sothat total depth of the bed of fuel and ash in each can be from 15 to 25feet. Also connected to the tops of the cylinders are outlet pipes 6 forproduct gas. The pipes are connected to a main 7 and are provided withvalves 8 and 9 for closing them alternately. Also connected to the topsof the two producers are inlet pipes 11 which may be connected to acommon supply pipe 12 for delivering steam to the producers. The inletpipes are provided with valves 13 and 14 which are closed alternately.

Extending up through the ash pans and into the lower ends of the gasproducer cylinders are the opposite ends of a cross-over conduit 16 thatmay have vertical end portions connected by a horizontal intermediateportion. Mounted a short distance above each end of this conduit is ahood 17 to prevent coal or ash from entering the conduit. Extending upthrough each, vertical end portion of the conduit and projecting abovethe adjoining hood is a blast pipe 18 which has a hood 19 located ashort distance above it. Below the cross-over conduit the blast pipesare provided with valves 21 and 22 and maybe connected by a commonsupply pipe 23 to a Source of oxygen-containing gas under pressure. Thisgas may be air or a gas containing a larger proportion of oxygen, evennearly pure oxygen. The steamzqs'upply pipe 12 and the lower gas supplypipe 23 are connected by a throttle valve 24 so that some of theoxygen-containing gas can be fed into the steam pipe to mix with thesteam. v

To start this apparatus in operation where a calting bituminous coal isto be gasified, the two producers first are filled with coke to a depthof from to feet. Valves 8, 14, 21 and Marc closed and valves 9, 13 and22 are open, sov that thesteamat a temperature of about 250 F., willenter the top of producer A and flow down- Ward through the coke bedtherein and then pass through the cross-over conduit 16 into the bottomof producer B. At the same time oxygen-containing gas, nearly pureoxygen for example, will flow into the bottom of producer B through itsblast pipe and producer B, after ignition, will start to operate withthe gases flowing upward through the coke bed and out through valve 9into the product main. The product gases flowing upward through the tallfuel bed exchange heat with it, heating the coke while they are cooledby it. After a suitable interval of time, valves 9, 13 and 22 are closedand valves 8, 14 and 21 are opened. The steam now flowing downwardthrough the coke inproducer B ispreheated by it, thereby reducing thetemperature of the coke, before entering the bottom of producer A by wayof the crossover conduit. Aftcr'p'roducer A'has been started inoperation, the entering preheated s'tearn makes it possible to reducethe oxygen flow through the blast'pipe below the original rate. Theproduct gases now flow upward through the coke bed in producer A, givingoif heat to the coke and leaving through valve 8. Switching of thevalves is repeated at intervals, whereby each unit is operatedalternately as a steam preheater and as a gas producer. After a numberof such cycles the tall fuel bed above the gasifica-tion zone in eachcylinder tends to take on an approximately linear temperaturedistribution that decreases from about 2000 F. at the top of thegasification zone to about 350 F. at the top of the bed. When thisoccurs it is time to start feeding the bituminous coal into the tops ofthe two producers.

The coal can be fed continuously or intermittently. If intermittent feedis used, it should be frequent enough to allow only a slight variationin the height of the beds so as to assure efficient cooling of theproduct gas. After the coal feed has been started, throttle valve 24 isopened and adjusted so as to allow a portion (say 10 to 50%) of thetotal oxygen used to mix with the steam flowing throughpipe 12. Bymixing a portion of the oxygen with the steam entering the tops of theproducers, the coal is exposed to an oxygen-rich atmosphere during everyhalf cycle.v This downwardly flowing atmosphere produces slow oxidationof the fusible constituents of the coal so that the coal loses itscaking properties before it sinks far enough down in the bed to reachthe temperature range of about 750-950 R, which is a common range inwhich fusion begins. The rate of oxidation is a function of the type ofcoal, the size of the coal particles, the oxygen content of thesurrounding gas and the temperature.

The proportion of oxygen mixed with the steam entering the top of thebed should be as large as possible for most effective destruction of thecaking properties of the coal. On the other hand, the oxygen content ofthe steam must be limited to that amount which will just avoid settingup of a self-sustaining gasification reaction at the bottom of theproducer which is in preheating service.

As the gasification proceeds, consuming fuel from the bottom of the bed,each coal particle gradually moves downward and is exposed to regions ofsuccessively higher temperatures. By using a tall bed of coal, the coalis exposed to oxygen-rich gas for a long time composed of manypreheating half-cycles. As an example, with a gasification rate of 25pounds of coal per hour per square foot of cross section of bed, thecoal moves downward at an average rate of about 0.25 foot per hour,moving 0.5 foot per hour during the gasitieation half-cycleand" beingalmost stationary during the preheating and oxygen exposure half-cycle.If the height of coal above the temperature zone of 750-950 F. is sixfeet, an exposure time of 12 hours to oxygen-rich gas is allowed beforethe fusion temperature is reached. Under these conditions the coal doesnot cake, and each coal particle remains a separate entity which isgradually transformed into coke during its passage down through theproducer without formation of large lumps or .blowholes.

The height of the coal bed should be adjusted for the type and size ofcoal being used to allow sufiicient oxygen exposure time to avoid cakingdifficulties at the normal fusion temperature. Alternatively, the coalmay be crushed to a size that will give satisfactory operation with thebed height desired to be used. However, with some coals, if the bedheight is small, the particle size required may be so small as torequire reduction of the allowable flow rates in order to keep theproduct gas from carrying away the coal particles.

The use of a deep bed of coal also results in unusually high thermaletficiency. The regenerative heat transfer effect of the two coal bedsallows preheating of the coal, the steam and a portion of the oxygenfeed by-heat recovered entirely from the product gas. For example, it iscalculated that with steam entering at 250 F. and with coal and oxygenentering at F. the product gas will leave the producer at a temperaturebetween 400 and 450 F. The switching time is adjusted to produce a riseand fall of temperature in the coal beds consistent with good heattransfer. Thus, with a fuel bed fifteen feet high above the gasificationzone, and a gasification rate of 25 pounds of coal per hour per squarefoot of bed cross section, the switching time per half-cycle may be fiveminutes This allows a temperature swing of about 16 If the switchingtime is increased, the temperature swing also will increase. Anincreased temperature swing will give poorer heat transfer but willproduce less wear on the switching valves. In some cases, switchingtimes as long as 30 minutes per half'cycle may be economical. V

For certain grades of bituminous coal which are fed to the producers inrelatively large lumps, it may not be possible to obtainsufiicientdestruction of their cal-:ing properties with a bed of reasonableheight. In such cases, each of the two producers can be provided withjet pipes to introduce a portion of the steam-oxygen mixture into thebed in a region just above the fusion zone. Such an arrangement is shownin Fig. 2, where a series of laterally spaced pipes 26 extend from aheader 27 through the side wall of a producer cylinder 1 and across itat a level just above the normal fusion zone. The undersides of thepipes are provided with small holes from which a mixture of steam andoxygen issues in jets that penetrate into the fused coal while it isstill in molten condition. The jets break up the material, preventingthe formation of large lumps which would impede the operation ofthe'producer, and provide a high oxygen content locally: for destructionof the fusible constituents of the coal. The total amount of oxygen usedis about the same as with the apparatus shown in Fig. 1, which is themaximum that can be used without sustaining gasificationat the bottom ofthe producer that is in preheating service.

This apparatus has the advantages of very good thermal eificiencyevenwhen the solid fuel is not a caking fuel, because the steam and 'thedown-flowing oxygencontainiug gas are preheated by the fuel beds whichremoved heat from the product gas. Thus, the product gas is cooled bythe fuelbeds to a lower temperature than heretofore, even if the bedsare no deeper than in the past. Also, if the fuel is non-caking, thethermal efficiency is improved even though no oxygen-containing gas isdelivered :to the top of the fuel beds, because the beds will becooled'inaterially by'the steam passing down through them. V l IAccording to the provisions of the patent statutes, I

have explained the principle of my invention and have illustrated anddescribed what I now consider to represent its best embodiment. However,I desire to have it understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallyillustrated and described.

I claim:

1. The continuous method of efiiciently gasifying solid fuel comprisingdirecting steam down through a hot first bed of said fuel to preheat thesteam, conducting the preheated steam from the bottom of said bed to thebottom of a second bed of said fuel while directing oxygen-containinggas up through the second bed to sustain combustion therein in order togenerate product gas, conducting the product gas away from the top ofthe second bed until the temperature of the top of that bed has beenraised a predetermined amount by heat from the product gas, thenstopping the withdrawal of product gas from the top and the delivery ofsteam and oxygen-containing gas to the bottom of the second bed,directing steam down through the hot second bed to preheat the steam,conducting the preheated steam from the bottom of the second bed to thebottom of the first bed while directing oxygen-containing gas up throughthe first bed to sustain combustion therein in order to generate productgas, conducting the product gas away from the top of the first bed untilthe temperature of the top of that bed has been raised a predeterminedamount by heat from the product gas, and continuing to reverse the cycleperiodically, whereby steamis preheated in each bed alternately by heatabsorbed by the bed from the product gas that has just left it.

2. The continuous method of efficiently gasifying caking bituminouscoal, comprising directing steam and oxygen-containing gas down througha previously heated first deep bed of said coal to preheat the steam andto reduce the tendency of the coal to cake, limiting the amount of saidoxygen to less than will sustain a gasification reaction at the bottomof said bed, conducting the preheated steam from the bottom of said bedto the bottom of a second bed of said coal while directingoxygencontaining gas up through the second bed to sustain combustiontherein in order to generate product gas, conducting the product gasaway from the top of the second bed until the temperature of the top ofthat bed has been raised a predetermined amount by heat from the productgas, then stopping withdrawal of product gas from the top and thedelivery of steam and oxygencontaining gas to the bottom of the secondbed, directing steam and oxygen-containing gas down through the secondheated bed to preheat the steam and to reduce the tendency of the coaltherein to cake, limiting the amount of said last-mentioned oxygen inthe same way as the first-mentioned oxygen delivered to the first bedconducting the preheated steam from the bottom of the second bed to thebottom of the first bed while directing oxygen-containing gas up throughthe first bed to sustain combustion therein in order to generate productgas, conducting the product gas away from the top of the first bed untilthe temperature of the top of that bed has been raised a predeterminedamount by heat from the product gas, continuing to reverse the cycleperiodically, whereby steam is preheated in each bed alternately by heatabsorbed by the bed from the product gas that has just left it, andadding coal to the top of the beds to maintain their depth.

3. The method of gasifying caking bituminous coal as defined in claim 2,in which some of said steam and oxygen-containing gas is conductedbefore heating directly to a predetermined zone of the bed in which thesteam is being preheated, said zone being the zone in which the coal isin molten condition.

References Cited in the file of this patent UNITED STATES PATENTS1,160,908 Koster Nov. 16, 1915 1,867,102 Russell July 12, 1932 2,131,696Brondegee et a1. Sept. 27, 1938 2,592,591 Odell Apr. 15, 1952 FOREIGNPATENTS 344,742 Great Britain Mar. 12, 1931

1. THE CONTINUOUS METHOD OF EFFICIENTLY GASIFYING SOLID FUEL COMPRISINGDIRECTING STEAM DOWN THROUGH A HOT FIRST BED OF SAID FUEL TO PREHEAT THESTREAM, CONDUCTING THE PREHEATED STEAM FROM THE BOTTOM OF SAID BED TOTHE BOTTOM OF A SECOND BED OF SAID FUEL WHILE DIRECTINGOXYGEN-CONTAINING GAS UP THROUGH THE SECOND BED TO SUSTAIN COMBUSTIONTHEREIN IN ORDER TO GENERATE PRODUCT GAS, CONDUCTING THE PRODUCT GASAWAY FROM THE TOP OF THE SECOND BED UNTIL THE TEMPERATURE OF THE TOP OFTHAT BED HAS BEEN RAISED A PREDETERMINED AMOUNT BY HEAT FROM THE PRODUCTGAS, THEN STOPPING THE WITHDRAWAL OF PRODUCT GAS FROM THE TOP AND THEDELIVERY OF STREAM AND OXYGEN-CONTAINING GAS TO THE BOTTOM OF THE SECONDBED, DIRECTING STEAM DOWN THROUGH THE HOT SECOND BED TO PREHEAT THESTEAM, CONDUCTING THE PREHEATED STREAM FROM THE BOTTOM OF THE SECOND BEDTO THE BOTTOM OF THE FIRST BED WHILE DIRECTING OXYGEN-CONTAINING GAS UPTHROUGH THE FIRST BED TO SUSTAIN COMBUSTION THEREIN IN ORDER TO GENERATEPRODUCT GAS, CONDUCTING THE PRODUCT GAS AWAY FROM THE TOP OF THE FIRSTBED UNTIL THE TEMPERATURE OF THE TOP OF THAT BED HAS BEEN RAISED APREDETERMINED AMOUNT BY HEAT FROM THE PRODUCT GAS, AND CONTINUING TOREVERSE THE CYCLE PERIODICALLY, WHEREBY STEAM IS PREHEATED IN EACH BEDALTERNATELY BY HEAT ABSORBED BY THE BED FROM THE PRODUCT GAS THAT HASJUST LEFT IT.